The invention relates to a liquid crystal display screen comprising a liquid crystal layer and two parallel substrates flanking said liquid crystal layer, and comprising means for influencing the transmission state of the liquid crystal layer, and a phosphor layer containing at least one phosphor, and comprising backlighting.
In liquid crystal display screens, use is made of the fact that by applying an electric field, the molecular orientation of several classes of liquid crystals can be controlled in such a manner that extraneous, incident, linearly polarized light is influenced in its direction of polarization. The different classes of liquid crystals include nematic, cholesteric and different types of smectic phases, which are each characterized by a different spatial arrangement of the molecules. For example, the widely used TN liquid crystal display screens (TN=twisted nematic) comprise nematic liquid crystals.
A conventional TN liquid crystal display screen is customarily made up of two glass plates whose inner sides are coated with a transparent electrode of indium tin oxide (ITO). A layer containing the liquid crystals is sandwiched between said glass plates. A 90xc2x0 rotated edge orientation between the two glass plates is imposed on the nematic liquid crystal molecules by orientation layers situated on the glass plates. This leads to a 90xc2x0 helix in the liquid crystal layer. Crossed polarizers on the outer surfaces of the glass plates and a two-dimensional backlighting complete the display screen. As long as no electric voltage is applied to the two ITO electrodes, the light originating from the backlighting, which is linearly polarized by the first polarizer, can follow the rotation through 90 degrees of the liquid crystal molecules and, subsequently, pass through the second polarizer; the display screen appears transparent. If a sufficiently high voltage is applied, the electric anisotropy of the liquid crystal molecules causes the helix to be removed and the direction of polarization remains uninfluenced; the polarized light cannot pass through the second polarizer, and the cell appears dark.
A complete picture is composed of a plurality of individual pixels, which are each driven via a matrix and function, as it were, as light valves. In colored liquid crystal display screens, a complete picture element is composed, like in CRT display screens, of three individually driven pixels for the colors red, blue and green. The color rendition in conventional liquid crystal display screens takes place by mosaic color filters which are pressed onto the front glass plate.
A drawback of the conventional liquid crystal color display screens comprising color filters resides in that the display screen can only be looked at from specific viewing angles, and external lighting is necessary, and the color saturation, luminous intensity and brightness are clearly inferior as compared to CRT display screens.
Liquid crystal color display screens comprising a phosphor layer have a higher luminous intensity and a larger angle of observation. For example, U.S. Pat. No. 4,822,144 discloses a liquid crystal color display screen which is operated in the transmission mode and based on a combination of liquid crystal switching elements and a phosphor layer, said phosphor layer being excited by a UV light source, and the brightness of the display screen being increased by an interference filter between the light source and the phosphor layer. The phosphor layer and the UV source may be situated at two remote sides of the liquid crystal switching elements. For the UV source use can be made of a mercury gas discharge lamp.
In a liquid crystal display screen where backlighting takes place using a mercury gas discharge lamp, the UV radiation generated is absorbed, however, by the material of the lamp bulb, the material of the liquid crystal layer and the substrates flanking said liquid crystal layer before said UV radiation reaches the phosphor layer.
U.S. Pat. No. 5,121,233 discloses a liquid crystal color display screen comprising a liquid crystal cell and backlighting consisting of an electron ray tube provided with a plurality of cathodes. The phosphor layer is arranged in the electron ray tube and consists of cathodoluminescent phosphors with pixels which are selectively excited.
A drawback of a liquid crystal color display screen with separate backlighting consisting of an electron ray tube with a plurality of cathodes resides in that the distance between the phosphor layer and the liquid crystal switching elements is large as compared to the lateral dimensions of the pixels. As a result, the light generated in the phosphor layer also illuminates regions beyond the intended regions, resulting in stray light being incident on the adjoining liquid crystal switching elements.
Therefore, it is an object of the invention to provide a liquid crystal display screen with improved backlighting.
In accordance with the invention, this object is achieved by a liquid crystal display screen comprising a liquid crystal layer and a first and a second transparent substrate flanking said liquid crystal layer, and comprising means for influencing the transmission state of the liquid crystal layer, and a phosphor layer, containing at least one phosphor in the form of phosphor dots, on the second substrate, and comprising a third substrate, which is arranged opposite the second substrate and connected with said second substrate so as to form a gastight gas discharge vessel filled with a filling gas, and comprising means for igniting and maintaining a dielectrically impeded discharge in the gas discharge vessel.
In this combination of a transmissive liquid crystal display screen and backlighting, liquid crystal cells and lighting are integrated into one unit. The dielectrically impeded discharges enable UV radiation of suitable wavelength to be two-dimensionally generated. Moreover, the phosphor layer can be provided in the immediate vicinity of the liquid crystal switching elements.
Instead of white light-illumination of color filters, phosphors are directly selectively excited in this liquid crystal display screen. This results in a substantially improved light output. By virtue of the compact design, this liquid crystal display screen can very suitably be used for laptops.
The service life of a backlighting with a dielectrically impeded discharge is comparable to that of UV phosphors employed for the same purpose, i.e. 50,000 hours.
Within the scope of the invention it is preferred that the distance d between the phosphor layer and the liquid crystal layer is smaller than the average radius of the phosphor dots.
Within the scope of the invention it may also be preferred that the filling gas contains an inert gas. In this embodiment, the absorption of the radiation generated by the dielectric discharge is negligibly small because there is no absorbing solid between the plasma and the phosphor layer, and the inert gas used also absorbs very little UV radiation.
It may also be preferred that the pressure of the filling gas ranges from 50 mbar to 10 bar. For this embodiment, pressure-proof containers can be dispensed with without the light output being affected.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.